The collagen network plays a critical role in determining functional properties of cartilage and other extracellular matrices in health, disease, aging, and development. However, it has not been possible to measure important physical properties of the collagen network independent of other tissue constituents within the extracellular matrix. Recently, we devised a new methodology to determine mechanical properties of the collagen network per se, such as its bulk modulus. This new approach entails (a) modeling the cartilage tissue matrix as a composite material consisting of two distinct phases (a collagen network and a proteoglycan (PG) solution trapped within it), rather than as a single solid-like phase; (b) titrating the equilibrium osmotic stress, and (c) using basic physical-chemical principles and independent experiments to determine the equilibrium retractive stress, Pc, developed within the collagen network, and the PG osmotic pressure, Ppg, as a function of tissue hydration or tissue volume. Specifically, during isotropic loading of a cartilage specimen in mechano-chemical equilibrium, Pc is given by the difference between Ppg and the applied osmotic stress. (This condition is analogous to the mechanical equilibrium in an inflated balloon, in which the retractive pressure exerted by the latex membrane equals the difference between the gas pressure inside and outside of the balloon). Then, by applying a known osmotic stress, and by determining Ppg from independent experiments, we calculate Pc at each resulting equilibrium hydration. In pilot studies, we used this approach to determine Pc vs tissue hydration in several normal human cartilage samples, in native and in trypsin treated normal human cartilage specimen, as well as in cartilage specimen from osteoarthritic (OA) joints. In both normal and trypsin-treated specimen, these curves coincided, showing a steep increase in Pc with increasing tissue hyration. In OA specimen, however, these curves are shallower, indicating that the collagen network is more flaccid (i.e., less stiff). Our findings highlight the role of the collagen network in limiting normal cartilage hydration, and in ensuring a high PG concentration in the matrix, both of which are essential for effective load bearing in cartilage, but which are lost in OA. These data also suggest that the loss of collagen network stiffness, and not the loss or modification of PGs may be the incipient event leading to the disintegraton of cartilage in OA. - cartilage, swelling, osteoarthritis, stiffness, aging, composite